An inkjet printing method is advantageous because it enables high-speed printing, makes almost no noise at the time of printing, enables direct printing on regular paper and does not require a fixing process so as to enable downsizing of a printer. Owing to these advantages, commercialization of the inkjet printing method is increasing. The inkjet printing method includes: a method which utilizes an electric/mechanical converter for jetting an ink droplet from a nozzle by making use of a motion caused by mechanical changes induced by input signals; and a so-called thermal inkjet method employing electrothermal transducers (heating resistances) for discharging an ink droplet by a pressure of bubbles generated on the heating resistances which generate heat upon application of a voltage pulse.
A known ink discharge method of an inkjet printer is to heat resistances or resistors by electric power applied to a printhead and discharge ink from a micro-nozzle by utilizing bubbles generated within the nozzle serving as an ink channel. In this case, to drive a printhead for discharging ink, a constant DC voltage is applied to the resistances to turn on/off switch devices connected in series to the resistances, thereby supplying the amount of power necessary for ink ejection to the heater resistances.
The printhead of an inkjet printer, which has a removable configuration, is held in a carriage unit moving in accordance with a width of a print medium, e.g., paper, at the time of printing. Therefore, a printhead set in a printer is not always the same. For instance, a printhead for printing black and white images, a printhead for printing color images, and so on, may be used for its purpose.
Since an arbitrary printhead is mounted as described above, the amount of head driving power necessary for discharging ink in a single discharge operation is controlled in order to stabilize the printing operation regardless of a variation in resistance values of the heater resistances in the printhead. Conventionally, the amount of electric power is controlled by detecting a variation of the heater resistance values based on a resistance value of a detection resistance, provided within the printhead that includes the heater resistances, then inputting the variation data to a control circuit provided on a main board fixed to a printer main body, and adjusting a head driving pulsewidth transmitted from the main board to the printhead. Conventionally, the amount of electric power is controlled by detecting a variation of the heater resistance values based on a resistance value of a detection resistance, provided within the printhead that includes the heater resistances, then inputting the variation data to a control circuit provided on a main board fixed to a printer main body, and adjusting a head driving pulsewidth transmitted from the main board to the printhead.
Furthermore, an amount of heater driving power is also controlled by detecting a temperature rise in a printhead with the use of a thermometer, provided within the printhead that includes the heater resistances, and adjusting a head driving pulsewidth transmitted from the main board to the printhead.
Note that the DC voltage applied to the heater resistances is supplied as a constant voltage from an AC adapter or a DC power source device provided within a printer.
FIG. 7 is a block diagram showing a brief construction of an example of a conventional inkjet printer. In FIG. 7, reference numeral 51 denotes an inkjet printhead; 52, a head carriage circuit board; 53, a head carriage; 54, a flexible cable; 55, a main board of the printer main body; 56, a driving pulse control circuit included in the main board 55; 57, a power source; and 58, a host apparatus.
The inkjet printhead 51, having a plurality of heating resistances, performs printing by discharging an ink droplet from a nozzle by making use of a pressure of bubbles, generated by converting energy to heat, the energy being supplied from the power source 57 in accordance with controlling of the driving pulse control circuit 56 in the main board 55.
The main board 55 converts an image signal, transmitted from the external host apparatus 58, to a bit signal which turns on/off each of the heating resistances in accordance with, for instance, a print mode or the like, and transmits the bit signal to the driving pulse control circuit 56 for generating a driving pulse. The driving pulse consists of, e.g., a heat source selection signal, printing serial signal, and so forth. The pulsewidth of the driving pulse is changed in accordance with information, such as a temperature of the inkjet printhead 51, so as to perform most appropriate ink droplet discharge.
The generated driving pulse is transmitted to the head carriage 53 through a movable cable such as the flexible cable 54, and transmitted to the inkjet printhead 51 through the head carriage circuit board 52. The inkjet printhead 51 is constructed with one or more removable head units. The head carriage 53 is structured such that it is movable. The head carriage circuit board 52 mainly serves as a relay for electrically connecting the flexible cable 54 with the inkjet printhead 51.
The power source 57 adopts an AC/DC converter having plural outputs for supplying a power source voltage to logical circuits such as the main board 55, motors (not shown), and inkjet printhead 51. Voltage precision is required particularly for the voltage supplied to the inkjet printhead 51, in view of an influence of a voltage drop caused by wiring resistances generated as a result of passing through the long flexible cable 54 and also for stable ink droplet discharge.
FIG. 8 is an explanatory view of connection between heating resistances and driving switches in the example of the conventional inkjet printhead.
In FIG. 8, reference numeral 16 denotes a heating resistance; 17, a driving switch; and 18, a power source line connected to a power source. Reference numeral 19 denotes a heating resistance driving circuit connector. One end of the heating resistance 16 is connected to the power source line 18 which receives voltage supplies from the power source, and the other end is connected to the driving switch 17.
Assume herein that the inkjet printhead has 64 nozzles. One end of the heating resistance 16, corresponding to each of the 64 nozzles, is connected to the power source line 18 which supplies a driving voltage, while the other end of the heating resistance 16 is connected to the driving switch 17. The heating resistance driving circuit connector 19 is connected to a heating resistance driving circuit (not shown) for being controlled such that a current is sent only to the heating resistances 16 selected in accordance with the heat source selection signal or printing serial signal transmitted from the main board. Note in FIG. 8, nozzles are numbered (N#1 to N#64) from the left.
FIG. 8 shows an example in which the 64 nozzles are divided into 8 blocks each having 8 nozzles, and nozzles are driven in block unit. In FIG. 8, nozzles N#1 to N#8 are included in block 1, N#9 to N#16 are in block 2, . . . , and N#57 to N#64 are in block 8.
Depending on an image to be printed, 8 nozzles in each block may be driven simultaneously. Among the signals outputted from the driving pulse control circuit 56, the heat source selection signal is used for determining a block to be driven in the 8 blocks, and the printing serial signal is used for selecting a nozzle discharging ink from the 64 nozzles. The amount of current sent through the power source line differs in accordance with the number of nozzles driven simultaneously. Therefore, even in a case of driving one block, a voltage drop level caused by wiring resistances is different depending on the number of nozzles driven in the block. Also, a sudden variation in the amount of current affects the voltage drop level.
As mentioned above, a voltage drop level differs in accordance with the number of nozzles driven in each block. Conventionally, the voltage drop level is corrected by controlling a driving pulsewidth so as to supply uniform heating energy (power) to the heating resistances of the nozzles. This construction is disclosed in, e.g., Japanese Patent Application Laid-Open No. 9-11463.
According to a conventional printhead driving method, a DC voltage for driving a printhead is supplied to the printhead through a flexible board, which connects the main board with a movable carriage board. The flexible board has a long wiring structure because it requires at least a length corresponding to the stroke of printhead's movement. Supplying a DC voltage for driving the printhead through such long wiring causes a problem of a voltage drop due to a wiring impedance. Because a head driving current is increasing in response to demands for high-speed and high-quality printers, an influence of the aforementioned voltage drop has come to the fore.
Furthermore, as means to control the amount of head driving power necessary for discharging ink in a single discharge operation, a method of adjusting a driving pulsewidth in accordance with a state of a printhead is adopted. However with this method, it is necessary to secure a maximum time width for a pulsewidth driving the heaters so as to make correction on the pulsewidth in accordance with various factors. This causes a problem of limiting the number of nozzles which can be used for printing per unit time, and as a result, limiting printing speed.
In addition, as mentioned above, a level of voltage drop caused in accordance with the number of nozzles driven in each block is corrected by controlling a driving pulsewidth so as to supply uniform energy to heating resistances of the nozzles. However, according to this method, it is controlled such that a driving pulsewidth is increased when a large number of nozzles is driven simultaneously. This makes a pulsewidth large (in other words, long time), holding from increasing the speed of an inkjet printer.